Websites setting Access-Control-Allow-Origin HTTP header

Controls origins (websites) that are allowed to load data from this web service over JavaScript-based APIs as part of Cross-Origin Resource Sharing (CORS) standard. By default, a web browser will refuse to load data over XmlHttpRequest from a website that is not in the same origin, which is a precaution against various types of data stealing attacks. The target server has to explicitly allow the origin domain using the Access-Control-Allow-Origin (ACAO) header, or it may allow all origins to access it using a wildcard *. The latter however creates a potential security issue if the website in question is transactional and processing sensitive data, so the wildcard should be only used on websites consciously offering public APIs.